1. Field of the Invention
The invention relates to cast parts having high impact and wear resistance made of white, at least predominantly austenitic, chromium cast iron, and to the process for producing cast chromium containing white irons which have high impact and wear resistance.
2. Description of the Prior Art
It is known to provide wearing parts which are exposed to high abrasion and impact wear from chromium cast iron, as disclosed in, for example, DIN 1695, BSI 4844, Part 3, ASTM A532-A and others.
The chemical composition of alloys described in the above specifications are usually within the following limits, wherein the values listed are percent by weight:
______________________________________ Carbon: 2.2 to 3.6% Silicon: 0.4 to 1.5% Manganese: 0.4 to 1.0% Nickel: 0.2 to 2.0% Chromium: 12.0 to 28.0% Molybdenum: 0.0 to 3.0% Vanadium: 0.0 to 1.0% ______________________________________
In the casting state, the basic mass consists of austenite and martensite with 10 to 50%, by weight, pearlite. The eutectic or primary carbides present, as determined by chemical analysis, which are embedded in the basic mass, constitute 18 to 40% of the structure.
As a result of the presence of pearlite or of secondary carbides, this basic mass is brittle and the endurance impact resistance as well as the abrasion resistance are low. By tempering, the basic mass is converted into martensite and the characteristics of impact resistance, abrasion resistance and the like, are improved. In the heat processed state, i.e., after hardening and single or multiple tempering and unstressing treatments, with the goal of reducing the residual austenite content, the hardness is increased and the endurance impact resistance is improved. The latter, however, still amounts to only a fraction of that which is known, for example, of the austenitic manganese steel, and consequently an occasional failure of the wearing parts occurs in operation. The operational fracture of wearing parts leads not only to a shut-down of equipment, e.g., a crusher, but sometimes to damage of the machinery itself. From the reported literature and laboratory experiments (for example, Kulmburg, Staska; Working Material Technique; 73, No. 1, pages 41-49: Diesburg, Borik; Symposium for the Mining Industry, Colorado, June 30, 1974, pages 15-41, etc.), it has been known that austenitic white chromium cast iron or cold-tough steels have good wearing characteristics and outstanding KI.sub.c (fracture resistance) values. The production of such structures, however, have been possible previously only by tempering from temperatures above 1100.degree. C. These high temperature tempering treatments cannot be profitably achieved, however, through the use of average furnaces.
It is an object of the invention to provide a process for producing cast parts having high impact and and wear resistance.
It is an object of the invention to provide predominantly austenitic chromium cast iron exhibiting, even in the cast state, an impact and wear resistance which will be sufficient for the parts, for example, of a crusher, exposed to shock and impact wear.